The present invention relates to a device and method for cool drying a gas.
Cool drying is applied for example in compressors that supply a hot compressed gas that is often saturated with water.
This compressed gas first has to be dried before it can be supplied to a pneumatic network because the moisture in the gas can be harmful to the components and tools in the pneumatic network as moisture can lead to corrosion or the accumulation of water in tools that are not designed for that purpose.
Cool drying is based on the principle that by cooling a gas that is saturated or partly saturated with water, moisture is removed from the gas as the moisture condenses and the condensed water is removed, after which the gas is again heated up whereby it is no longer saturated and is thus dryer.
For cool drying, a device is used that primarily consists of a closed cooling circuit that contains a coolant that can be circulated in the circuit by a compressor and which further contains, successively in the direction of flow of the coolant, a condenser connected to an outlet of the compressor; an expansion means followed by an evaporator connected to an inlet of the aforementioned compressor, whereby the evaporator forms the primary part of a heat exchanger with a secondary part through which the gas to be dried is guided.
As a result of the evaporation of the coolant in the evaporator, or thus the primary part of the heat exchanger, as is known, heat is extracted from the gas to be dried flowing through the secondary part, whereby this gas to be dried is cooled and, after evacuation of the condensate formed, is heated up again.
Such a device is designed for a nominal load for drying a normal gas flow.
In the unloaded state, in other words when no gas to be dried is flowing through the heat exchanger, the cooling capacity of the cooling circuit is too high, such that freezing can occur in or after the evaporator, which must absolutely be avoided.
Moreover the coolant in the cooling circuit is continually pumped around without the cooling capacity of the cooling circuit being usefully employed for drying the gas, such that a lot of energy is lost.
A known solution to this is to provide a bypass pipe in the closed cooling circuit with a bypass valve in it, which, on the one hand, is closed when the device is loaded, or in other words when a gas to be dried flows through the heat exchanger and, on the other hand is open when the device is unloaded and thus no gas to be dried flows through the heat exchanger.
In loaded situations the bypass pipe thus has no effect as it is then closed, and as a result of this the cooling circuit operates at full capacity as the entire flow of coolant that is compressed by the compressor is also guided through the condenser and the expansion means, which together ensure considerable cooling of the coolant, and as this full flow of cold coolant also flows through the evaporator in the heat exchanger to cool the gas to be dried.
In the unloaded state on the other hand the open valve ensures that the compressor is bypassed and that, as a result of this, at least a part of the coolant compressed by the compressor does not flow through the condenser and the expansion means and this part of the coolant is thus not cooled by expansion such that there is much less cooling in the evaporator and thus there is much less risk of freezing in or downstream of the evaporator.
Downstream and upstream are considered in the direction of flow of the coolant in the closed cooling circuit.
For a bypass valve that can fulfil the aforementioned functions, it is known to use a type of valve known by the name of ‘hot gas bypass’ (HGBP), and more specifically this type of valve is a pressure-controlled bypass valve with a valve element that is held in a closed position under the influence of a spring element to close the bypass pipe, and which is also provided with a pressure-sensitive element that acts on the aforementioned valve element, and which, when it is subject to a sufficient pressure drop, can open the valve against the spring force.
In the known devices, the pressure-sensitive element is exposed to a control pressure that is tapped off locally from a point downstream of the evaporator via an internal control pressure pipe in the circuit, more specifically at the point where the bypass pipe leads into the cooling circuit.
When such a device changes from an unloaded state to a loaded state through a quantity of gas to be dried suddenly being driven through the heat exchanger, the temperature in the evaporator increases due to the heat transfer from the gas be dried to the cooler coolant in the evaporator, such that more coolant evaporates in the evaporator, which itself increases the pressure in the evaporator and also downstream of the evaporator, and this pressure increase is felt by the bypass valve which will thereby close when the pressure increase is sufficient.
A feature of the control characteristic of an HGBP valve is a given pressure difference of the control pressure between the unloaded and loaded state, which in known devices in a loaded state results in a relatively high pressure of the coolant at the outlet of the evaporator, and thus also in the evaporator where the pressure is even higher as a result of the frictional losses that occur when coolant flows through the evaporator.
As the coolant in the closed cooling circuit is in a two-phase region, there is an unambiguous relationship between the pressure and temperature of the coolant: a higher pressure in the evaporator also implies a higher temperature in the evaporator.
This leads to the disadvantage that the gas to be dried is not cooled optimally for drying by the evaporator, which results in a higher pressure dew point of the gas at the outlet of the secondary part of the heat exchanger, whereby at this outlet the aim is for the lowest possible temperature (the lowest temperature of the gas to be dried is also called the LAT or ‘Lowest Air Temperature’—although the term LAT is also used for gases other than air) and corresponding pressure dew point of the gas for good drying.